The magnetic properties of rare earth-transitional metal-boron alloys such as NdFeB-type alloys are well known to those in the art. One of the applications in which NdFeB alloys are used is the production of bonded magnets. Bonded magnets consist of magnetic particles agglomerated in a binder, such as an organic polymer, and exhibit strong magnetic properties.
NdFeB alloy powders for use in the production of bonded magnets have been commercially prepared by crushing melt-spun ribbons into powder. The flake-like particles formed by crushing melt-spun ribbons generally exhibit isotropic behavior and relatively poor flowability. Consequently, they do not achieve their full potential as magnetic materials and are somewhat difficult to form into bonded magnets using conventional injection molding equipment. In addition, the mechanical strength of bonded magnets formed of such flake-like particles is relatively poor because of stress concentrations arising from the sharp edges of the flake-like particles.
NdFeB alloy powders have been prepared by crushing and pulverizing cast ingots of NdFeB alloys. Powders prepared in this manner typically display intrinsic coercivity, H.sub.ci, values of less than 5 kOe because of their large-grained microstructures formed during relatively slow cooling and metallurgical defects or oxidation on the particle surfaces. As a consequence of the low H.sub.ci values they display, crushed and pulverized NdFeB alloy powders have not been used in the preparation of bonded magnets.
Hydrogen processing of NdFeB alloys in ingot and powder form is described in U.S. Pat. No. 4,981,532, to Takeshita et al. and a publication by I. R. Harris and P. J. McGuiness ("Hydrogen: its use in the processing of NdFeB-type magnets and the characterization of NdFeB-type alloys and magnets," Proceedings of the Eleventh International Workshop on Rare Earth Magnets and Their Applications, October 1990, Carnegie Mellon University Press, Pittsburgh, Pa.). Using a technique known as hydrogen disproportionation, desorption, and recombination (HDDR), coercive NdFeB alloy powders have been prepared by heating an NdFeB alloy in a hydrogen atmosphere and removing the hydrogen in a desorption step. Powders prepared by subjecting a cast NdFeB alloy, either in ingot or powder form, to HDDR have irregularly shaped, i.e. non-spherical, particles with the shape of the particles varying depending upon the fracture patterns in the alloy. Generally, NdFeB powders prepared by subjecting a cast alloy to HDDR are isotropic, although some anisotropic behavior has been noted for cast alloys containing a refractory metal addition such as Nb, Ti, Zr, or Hf.
It is known that spherical NdFeB alloy powders can be produced using gas atomization. In principle, a spherical powder morphology is well suited for use in the production of bonded magnets because the relatively high flowability of spherical powders is conducive to injection molding. Furthermore, the mechanical strength of bonded magnets formed from spherical particles should be high because the spherical shape of the particles minimizes the possibility that stress concentrations from sharp-edged particles will occur during bending. Nevertheless, spherical NdFeB alloy powders produced by gas atomization have not been widely used in the production of bonded magnets because they display low H.sub.ci values.
A method for improving the intrinsic coercivity of relatively coarse spherical NdFeB alloy powder produced by gas atomization is disclosed in U.S. Pat. No. 5,127,970, to Kim. The method involves subjecting a spherical NdFeB alloy powder having a particle size within the range of 200-300 microns to dual hydrogen absorption-desorption treatment cycles at an elevated temperature in the range of 660.degree. C. to 850.degree. C. While the intrinsic coercivity of the NdFeB powder is enhanced, the nature of the powder remains isotropic. Thus, the enhanced remanence (B.sub.r) and maximum energy product (BH.sub.max) desired for commercial applications, which result from anisotropic behavior, are not realized.
Accordingly, it is the primary object of the present invention to provide a spherical magnetic particle that is magnetically anisotropic.
An additional object of the invention is to provide a magnetic material of high intrinsic coercivity. A further object of the invention is to provide a bonded magnet formed from anisotropic spherical particles having a high coercivity per particle.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.